Hi
My desktop computer doesn't recognize my Fiio x3 at all.I have no problems with other divices.
I have tested it on another computer and still the same problem.
My firmware is FW2.1 and the X3 recognize the sd card.

Do you mean recognising the x3 as a card reader or as a DAC? You need to set the correct USB mode in x3 's System Settings before connecting the x3 to the computer.